Regulatory safety requirements specify limits and regulations in terms of output power, rather than in terms of output current or output voltage. For example, Underwriter Laboratories (UL) 62386-1 classifies electronic products as “PS2 Products” when the output power delivered by the products is limited to under one hundred watts. UL tests the PS2 circuit, and the PS2 circuit must shut down its output within five seconds of the application of a hundred watt load.
Many PS2 devices monitor their output power by including a current sensing device in the power delivery path, and shut down when the sensed current exceeds a threshold that indicates that the output power is at or near the limit. The current (e.g., in amps) is equal to the power (e.g., in watts) divided by the voltage (e.g., in volts). Thus, as an example, a PS2 product with a 24 volt input power supply might be designed such that the current monitor circuit trips at no more than 4.16 amps (i.e., 100 watts/24 volts).